This invention relates to feedback canceling circuits and, in particular, to a circuit for precisely correcting the amplitude of a component signal for removing the component signal from a composite signal.
Hearing aids, public address systems, telephones and other devices are often plagued by feedback. Sometimes the feedback is simply an annoying echo, other times the feedback is sufficient to cause the circuit to squeal or oscillate, often loudly. As described in U.S. Pat. No. 5,649,019 (Thomasson), the disclosure of which is incorporated by reference, a difficulty with detecting an echo is determining whether or not a signal is an echo and another difficulty is determining the travel time of the echo.
As described in the Thomasson patent, these difficulties are overcome by tagging original sound with an inaudible replica of the sound and detecting the tag in the returned signal. The system for doing this includes two channels, one of which corrects for phase and amplitude shifts between the channels. One channel recovers the original signal from the tag while the other channel removes the tag from the returned or composite signal. The recovered signal, or component, is subtracted from the composite signal to eliminate echo in the composite signal. In order to cancel an echo, the amplitudes and phases of the signals must be matched and the Thomasson patent describes circuitry suitable for this purpose.
In some applications, particularly low noise environments, it is desirable to match amplitudes exactly to ensure complete cancellation of the echo and the circuits of the prior art are not sufficiently precise for this purpose.
For many applications, it is desired to have the electronics as small as possible, e.g. in telephones or communication equipment in general. If size were no object then it would be relatively easy to provide suitable filters, multipliers, and so on for matching phase and amplitude. It is preferred to integrate the electronics as much as possible, which does not mean that the problem is solved. Rather, the problem is moved from the telephone to the wafer, where as small a die size as possible is desired for reduced costs.
In view of the foregoing, it is therefore an object of the invention to provide an improved echo canceling circuit.
Another object of the invention is to provide a circuit for exactly matching the amplitudes of two signals.
A further object of the invention is to provide a low noise circuit for removing a component from a composite signal.
Another object of the invention is to provide a circuit amenable to integration in relatively small size on a semiconductor die.
The foregoing objects are achieved in this invention by subtracting the component signal from the composite signal to produce a remainder signal, correlating the remainder signal with the component signal to produce a product signal, averaging the product signal, and adjusting the magnitude of the component signal in accordance with the averaged product signal to minimize the product signal. The amplitude of the component signal is adjusted in a programmable gain amplifier controlled by an up-down counter. The up-down counter is part of a digital control loop including a pseudo-multiplier for multiplying the remainder signal with the component signal. The output of the multiplier controls the direction of the count, which is generally continuous except that it cannot roll over or roll under. The remainder signal is the received signal with the echo removed.